For carriers or supports for catalysts used under high-temperature and -humidity conditions, for instance, catalysts for high-temperature combustion such as catalysts for combustion used with gas turbines, kerosine and gas heaters, petroleum and gas hot water-supply equipment, heavy oil combustion equipment and reformers for fuel cells, catalysts for steam reformers and catalysts for the purification of odors and exhaust gases, cordierite (2MgO-2Al.sub.2 O.sub.3 -5SiO.sub.2), mullite (3Al.sub.2 O.sub.3 -2SiO.sub.2) and the like have heretofore been used as ceramic substrates. However, they are coated on their surfaces with a porous material for use, since they are of no porosity. As the porous material, use is generally made of alumina, which carries thereon a catalytically active metal component and is used as a catalyst. However, a problem with alumina is that when used under high-temperature conditions, alumina is transformed into .alpha.-alumina, which is so decreased in specific surface area that the active metal component becomes poor in dispersibility and is rapidly deactivated.
It has also been reported that lanthanum .beta.-alumina (LaO.sub.3 -11Al.sub.2 O.sub.3) and barium alumina (BaO-6Al.sub.2 O.sub.3) are of high heat resistance (see Japanese Patent Laid-Open Publication No. 62-153158). However, they offer problems that their specific surface areas after high-temperature firing or calcination are less than satisfactory, and their commercial or industrial use incurs cost rises due to an increase in the amount of a noble metal to be used as a catalyst component.
Further, it has been reported that an aqueous solution of a mixture of an alumina hydrate with colloidal silica is thickened and gelled by the addition of a basic compound, and the resulting gel is dried and aged into a heat-resistant silica-alumina base catalyst carrier composition (see Japanese Patent Laid-Open Publication No. 61-287446). Still further, there has been a report on a monolithic catalyst support comprising a structural phase of a sintered ceramic material and a high surface area support phase of a porous inorganic oxide integral with the structural phase, wherein said porous inorganic oxide support phase consists essentially of 50% by weight to 93% by weight of alumina and 7% by weight to 50% by weight of silica. However, a problem common to these compositions is that when used at high temperatures for an extended period of time, their specific surface areas are sharply decreased. Another problem with said monolithic catalyst support is that even when it is intended to enhance a coating layer for, e.g., a honeycomb structure, it is difficult to form a uniform and thick coating layer within close cells of honeycomb.